1. Field of the Invention
This invention generally relates to a transmitter-receiver for use in mobile units, for example, a vehicle or a ship, and, more particularly, to a transmitter-receiver for use when measuring the distance and the relative velocity between such mobile units.
2. Description of the Related Art
There has been developed what is called an automobile millimeter-wave radar which aims at measuring the distance between a vehicle and another vehicle running in front or rear thereof while the vehicles are running on a road. Generally, such a radar comprises a transmitter-receiver is produced in a module composed of a millimeter-wave oscillator, a circulator, a coupler, a mixer and an antenna, and is mounted on a front or rear portion of a vehicle.
For instance, as shown in FIG. 16, a truck measures the distance therefrom to a passenger car running in front thereof and the relative velocity therebetween by transmitting and receiving millimeter waves in accordance with a frequency modulated-continuous wave (FM-CW) method. FIG. 17 is a block diagram illustrating the configuration of the entire millimeter-wave radar. A transmitter-receiver and an antenna of this figure are mounted on a front portion of the vehicle or truck in the case of the example illustrated in FIG. 16. In contrast, a signal processing unit is usually provided at an arbitrary location in the vehicle. A signal processing portion provided in the signal processing unit is operative to extract as numerical information the distance therefrom to the vehicle, which runs in front thereof, and the relative velocity therebetween, as numerical information by using the transmitter-receiver. Further, a control-alarm portion is operative to issue an alarm according to the relation between the running speed of the vehicle or truck and the relative velocity thereof, for example, when predetermined conditions are met, or when the relative velocity thereof with respect to the vehicle running in front thereof exceeds a threshold value.
FIG. 18 is a schematic plan diagram illustrating the configuration of a prior art transmitter-receiver. In this figure, reference numeral 2 designates a circulator, on the two sides of which an oscillator 1 and a terminating device 3 are placed, respectively. Reference numeral 11 denotes a dielectric resonator that acts as a primary radiator for transmitting waves. Further, a dielectric strip 4 is placed between the circulator 2 and this dielectric resonator 11. Reference numeral 12 designates a dielectric resonator acting as a primary radiator for receiving waves; and 15 a mixer. Moreover, a dielectric strip 14 is placed therebetween. Moreover, a linear dielectric strip 6, curved dielectric strips 5 and 7, and terminating devices 8 and 9 are placed as illustrated in this figure.
Furthermore, a coupler 10 is provided by a proximity portion, where the dielectric strips 4 and 5, are close to each other. Additionally, another proximity portion, where the dielectric strips 14 and 7 are close to each other, provide a coupler 13. Further, dielectric lenses 16 and 17 are mounted on the upper portions of the dielectric resonators 11 and 12, respectively.
FIG. 19 is a diagram illustrating an equivalent circuit of the transmitter-receiver shown in FIG. 18. The oscillator 1 is provided with a varactor diode and a Gunn diode. Further, an oscillation signal outputted therefrom is transmitted or propagated to the dielectric resonator 11 through the circulator 2 and is then radiated through the dielectric lens 16. The circulator 2 and the terminating device 3 compose an isolator. An RF signal received through the dielectric lens 17 and the dielectric resonator 12 propagates the dielectric strip 14. A local oscillator (LO) signal is mixed into the dielectric strip 14 by the couplers 10 and 13 and is further inputted to a mixer 15. This mixer 15 is constituted by a Schottky barrier diode and generates IF (intermediate frequency) signals.
FIG. 20 is a schematic plan view of the transmitter-receiver in the case where a transmit/receive antenna is used in common for both transmitting and receiving. In this figure, reference numeral 2 designates a circulator. Further, an oscillator 1, a mixer 15 and a dielectric resonator 11 serving as a primary radiator are placed at ports of the circulator 2 through dielectric strips 4, 14 and 18, respectively. Furthermore, a coupler is configured by bringing a curved dielectric strip 19, whose both ends are terminated, close to dielectric strips 4 and 14.
FIG. 21 is a diagram illustrating an equivalent circuit of the transmitter-receiver shown in FIG. 20. A signal outputted from the oscillator 1 is radiated by the antenna, which is comprised of the dielectric resonator 11 and the dielectric lens 16, through the dielectric strip 4, the circulator 2 and the dielectric strip 18. Further, waves reflected from an object are inputted to the mixer 15 through the dielectric strip 18, the circulator 2 and the dielectric strip 14. The inputted waves are mixed by a coupler, which consists of the dielectric strips 4, 14 and 19, resulting in a mixed signal (RF signal+LO signal), and the mixed signal is inputted to the mixer 15 that is constituted by a Schottky barrier diode and is operative to generate IF signals.
Another type of transmitter-receiver for use in a millimeter-wave radar using a conventional nonradiative dielectric (NRD) waveguide is designed to use a NRD waveguide of the configuration illustrated in FIGS. 22A and 22B. In FIG. 22A, reference numerals 101 and 102 designate conductive plates, respectively. Further, dielectric strips 100a and 100b and a substrate 103 are placed between these two conductive plates. Further, by appropriately setting the distance between the aforementioned conductive plates, the size of the dielectric strips and their relative dielectric constant (or permittivity), the dielectric strip portions are established as propagating regions and the other regions become non-propagating regions (namely, blocking regions). For example, when the size or dimension of each portion and the relative dielectric constant are determined as shown in FIG. 23B, the transmission of signals in the propagating region is realized only in a certain range of frequencies, which are not less than a predetermined value, as is seen from phase constant characteristics illustrated in FIG. 23A.
However, LSM01 mode and LSE01 mode, which are basic transmission modes of an NRD waveguide, are orthogonal to each other, so that low-loss characteristics are exhibited in the case of a straight-line path. Nevertheless, in the case of a curved path (namely, in the curved strips described above), the orthogonality is lost and a coupling is caused between these modes. Thus, low-loss characteristics are obtained only in a range restricted by a radius of curvature and a bending angle. In the case of the waveguide having the dimensions shown in FIG. 23B, if the bending angle is, for instance, 60 degrees, characteristics, by which the loss is minimized, are obtained in the case where the radius of curvature is 36.3 mm. Further, if the bending angle is 90 degrees, characteristics, by which the loss is minimized, are obtained in the case where the radius of curvature is 22.5 mm. Therefore, the loss increases if the value of the radius of curvature is other than 36.3 mm when the bending angle is, for instance, 60 degrees. Thus, in the case of the conventional transmitter-receiver, the degree of freedom in designing the bend portion and in constituting the coupler by the bend portion is low. Consequently, the size of the transmitter-receiver is not reduced so much even when designing the transmitter-receiver in such a manner as to minimize the size of the bend portion and the transmission loss of the coupler.
On the other hand, the aperture diameter of an antenna is determined according to the specifications of a transmitter-receiver. Namely, in a condition in which the breadth of the major lobe of a radiation (or field) pattern of a transmitted beam (or wave) at a distance of 100 m in front of the antenna is not more than 3.5 m, the beam width is 2 degrees. For instance, it is necessary to set the aperture diameter of the radiator of the antenna at 170 mm. Further, in a condition in which the breadth of the major lobe of a radiation pattern of a transmitted beam at a distance of 50 m in front of the antenna is not more than 3.5 m, the beam width is 4 degrees. For instance, it is necessary to set the aperture diameter of the radiator of the antenna at 80 mm. Thus, the aperture diameter of the antenna is necessarily determined according to the specifications of the transmitter-receiver. As illustrated in FIG. 18, in the case of the prior art transmitter-receiver, the size of the region in which the elements such as the oscillator, the circulator and mixer are formed, is larger than the antenna size, so that the size of the entire transmitter-receiver cannot help becoming large.